Cascade control
Without the acceleration afforded by the second controller, the first controller would see the process becoming slower and slower. So while we desire to balance disturbance rejection and set point tracking performance for the inner secondary controller, in practice, SP tracking tests tend to provide the most direct route to validating inner secondary controller performance.
During closed loop control of the outer loop including during tuningmake sure that the inner loop axis remains in closed-loop control and geared to the PDIF output of the outer loop axis.
Industry Best-Practice Use of Cascade has grown dramatically across the process industries, and it has become best-practice to apply it to most Temperature and Level loops where a corresponding Flow loop is available.
The cascade block diagram is presented in the graphic below click for a large view and discussed in detail in this article. Inner loop must be measured and controllable.
The flow controller, in turn, drives a control valve to match the flow with the set point the level controller is requesting. A cascade arrangement should be tuned starting with the innermost loop. Software Provides Benefit Given that the outer primary controller design and tuning is based on the specifics of the inner secondary loop, a guess and test approach to a cascaded implementation can prove remarkably wasteful, time consuming and expensive.
In the example above the relatively slow level control loop is isolated from any control valve problems by having the fast flow control loop deal with these problems.
In the water heater example: Setpoint — temperature desired for the water in the tank Primary controller master — measures water temperature in the tank and asks the secondary controller for more or less heat Secondary controller slave — measures and maintains steam flow rate directly Actuator — steam flow valve Secondary process — steam in the supply line Inner loop disturbances — fluctuations in steam supply pressure Primary process — water in the tank Outer loop disturbances — fluctuations in the tank temperature due to uncontrolled ambient conditions, especially fluctuations in the inflow temperature Secondary process variable — steam flow rate Primary process variable — tank water temperature Challenges Cascade control can also have its drawbacks.
Cascade control ppt
Cascade Control Disadvantages Cost of measurement of the secondary variable assuming it is not measured for other reasons. In the original single-controller arrangement, a drop in the steam supply pressure would first have to lower the tank temperature before the temperature sensor could even notice the disturbance. In production processes with streams comprised of gases, liquids, powders, slurries and melts, disturbances are often unmeasured and beyond our ability to manipulate at will. P-Only, PI, etc. If that rate turns out to be insufficient to produce the desired tank temperature, the first controller can call for a higher flow rate, thereby inducing the second controller to provide more steam and more heat or vice versa. Thus, with our process steady at or as near as practical to its design level of operation DLO , we generate dynamic CO2 to PV2 process response data with either a manual mode open loop bump test or a more sophisticated SP driven closed loop bump test. This, in turn, impacts the design and tuning of the outer primary controller.A better way A cascade control system could solve both of these problems as shown in Figure B where a second controller has taken over responsibility for manipulating the valve opening based on measurements from a second sensor monitoring the steam flow rate.
That may sound like a convoluted way to achieve the same result as the first controller could achieve on its own, but a cascade control system should be able to provide much faster compensation when the steam flow is disturbed.
The decision to implement Cascade Control can be evaluated simply in terms of Pros and Cons.